Unconscious goal pursuit strengthens voluntary force during sustained maximal effort via enhanced motor system state

Abstract

Maximal voluntary force is known to be enhanced by shouting during sustained maximal voluntary contraction (MVC) via the enhancement of motor cortical excitability. However, whether excitatory input to the primary motor cortex from areas other than the motor-related cortical area induces muscular force-enhancing effects on the exertion of sustained maximal force remains unclear. Therefore, by examining motor evoked potentials to transcranial magnetic stimulation during sustained MVC and assessing handgrip force, the present study aimed to investigate the effects of subliminal goal-priming with motivational rewards on the state of the motor system. The findings revealed that when combined with rewards in the form of a consciously visible positive stimulus, barely visible priming of an action concept increased the maximal voluntary force and reduced the silent period (i.e., reduced motor cortical inhibition). To our knowledge, this is the first study to report a link between the muscular force of subliminal reward-goal priming during MVC and the enhancement of motor system activity through subliminal reward-goal priming operating on the motor system, possibly through the potentiation of activity of the reward-linked dopaminergic system.

Fig. 1

Experimental and priming procedures. (A) Experimental procedure. Each experiment comprised two tasks (pre-intermittent maximal voluntary contraction [MVC] and sustained MVC), which were spaced at least 3 min apart on the same day. The pre-intermittent MVC task was used as a baseline for the sustained MVC task, which consisted of three conditions (control, priming, and priming-plus-reward), each with a duration of approximately 237 s. Each condition was performed with an interval of more than 30 min. The total experimental time exceeded 90 min. The timing of transcranial magnetic stimulation (TMS) is indicated by the arrow. (B) Priming procedure. In the priming-plus-reward condition, the subliminal barely visible exertion primes were always paired with positive words. In the priming condition, although exertion primes and positive words were both displayed, they were never paired with each other. In the control condition, subliminal barely visible exertion words were never displayed. The order of possible stimulus pairs was randomized within each condition. Exertion, positive, and neutral words were shown in Japanese. Each trial in each condition began with a 1000-ms presentation of a random eight letter string (e.g., DZXLTOTM) as a forward mask. This was followed by the subliminal barely visible prime, displayed for 33 ms. A random letter string was again displayed for 100 ms as a backward mask, after which a consciously visible word was presented for 150 ms. Occasionally, a dot was presented for 33 ms (the dot was visible because of the absence of a backward mask), either above or below the neutral or positive word.

Fig. 2

Typical recordings of handgrip force, and background electromyography (bEMG) and of motor evoked potential (MEP) waveforms of the flexor carpi ulnaris during the maximal voluntary contraction (MVC) of handgrip in a pre-intermittent MVC task in a single participant. The timing of transcranial magnetic stimulation (TMS) is indicated by the arrow. The handgrip force declined when TMS was delivered during the contraction, the timing of which was different in each contraction. (A) Handgrip force, (B) bEMG, and (C) MEPs during handgrip MVC. The bidirectional arrows indicate amplitudes of MEP (blue) and the silent period (red).

Fig. 3

Typical recordings of handgrip force during the maximal voluntary contraction (MVC) of handgrip in sustained MVC task in a single participant. The timing of transcranial magnetic stimulation (TMS) is indicated by the arrow. Handgrip force produced by the superimposed twitch (twitch force) following TMS was expressed as a fraction of the pre-stimulus force at each TMS.

Fig. 4

Effects of unconscious goal pursuit on the maximal voluntary force of handgrip. The maximal voluntary force for each condition (control, priming, and priming-plus-reward) during the maximal voluntary contraction (MVC) of handgrip in the sustained MVC task. Barely visible goal-priming with motivational reward significantly increased the maximal voluntary force of handgrip and significantly decreased the maximal voluntary force produced by the superimposed twitch (twitch force) following transcranial magnetic stimulation (TMS) during the sustained MVC. (A) The maximal voluntary force for each condition (control, priming, and priming-plus-reward) during the maximal voluntary contraction (MVC) of handgrip in the sustained MVC task. Data are expressed as the mean ± standard error of the mean. (B) The twitch force for each condition (control, priming, and priming-plus-reward) during the maximal voluntary contraction (MVC) of handgrip in the sustained MVC task. Data were expressed as a fraction of the pre-stimulus force at each TMS (mean ± standard error of the mean). (C) (D) Data were expressed as a percentage of the maximal voluntary force produced in pre-sustained MVC task. Barely visible goal-priming with motivational reward significantly increased the maximal voluntary force during the sustained MVC. Bar height shows the average across participants, the error bar represents one standard deviation, and the curve shows the normal distribution curve of individual plots. ∗Statistically significant difference between conditions.

Fig. 5

Effects of unconscious goal pursuit on the silent period, motor evoked potential (MEP) amplitude, and background (b) EMG. (A) The silent period, (B) amplitudes of MEPs of the flexor carpi ulnaris (FCU), and (C) bEMG activity of the FCU for each condition (control, priming, and priming-plus-reward) during handgrip maximal voluntary contraction (MVC) in the sustained MVC task. Data are expressed as the mean ± standard error. (D) The silent periods for the three conditions during handgrip MVC in the sustained MVC task. (E) Averaged amplitudes of 50 MEPs of the FCU for the three conditions during handgrip MVC in the sustained MVC task are shown as a percentage of those of three MEP amplitudes (an average of three recordings) during handgrip MVC in the pre-intermittent MVC task. (F) bEMG activity of the FCU for the three conditions during sustained MVC are shown as a percentage of those of three bEMG activities (an average of three recordings) during handgrip MVC in the pre-intermittent MVC task. Barely visible goal-priming with motivational reward significantly shortened the silent period during sustained MVC. Bar height shows the average across participants, error bars represent one standard deviation, and the curve shows the normal distribution curve of individual plots. ∗Significant difference between conditions.

Fig. 6

Effects of unconscious goal pursuit on pupil area. (A) Time course of pupil area (dots) expressed as the mean for each condition (control, priming, and priming-plus-reward) starting at the onset of priming word presentation for the three conditions during the maximal voluntary contraction (MVC) of handgrip in the sustained MVC task and lasting until approximately 32 s after the sustained MVC (mean ± standard error of the mean). The data were low-pass filtered with a cutoff frequency of 1 Hz using a fourth-order Butterworth filter. The bidirectional arrow (↔) with “5 s” indicates the period of 5 s immediately before the word presentation in the sustained MVC task. (B) Averaged pupil area (dots) for 500 ms before each transcranial magnetic stimulation while squeezing a handgrip device for the three conditions during the MVC of handgrip in the sustained MVC task was expressed as a percentage of mean pupil area during the 5 s period immediately before the word presentation for the three conditions. Barely visible goal-priming with motivational reward significantly increased pupil area during sustained MVC. Bar height shows the average across participants, the error bars represent one standard deviation, and the curve shows the normal distribution curve of individual plots. ∗Statistically significant difference between conditions.